The extremely halotolerant black yeast Hortaea werneckii - a model for intraspecific hybridization in clonal fungi

Plant-growth promotion by halotolerant black yeast Hortaea sp. strain PMGTC8 associated with salt crystals from solar saltern of Goa, India

Plant-growth-promoting microbes are sustainable alternative to improve the soil fertility and plant-growth facilitating the nutrients uptake in normal and stressed environmental conditions. Among these, halotolerant and halophilic microorganisms with plant- growth-promoting ability are better candidates that could be exploited as bioinoculants in salinity affected agriculture soils. Considering this, the current study aimed to isolate, characterize and determine the plant-growth-promoting potential of the culturable halotolerant black yeast associated with salt crystals from the solar saltern of Goa, India. The results revealed 1.3 × 104 CFU/g of viable number of colonies on 25% NaCl Tryptone Yeast Extract (NTYE) agar after 30-45 days incubation. Among ten morphologically distinct isolates, a black pigmented strain PMGTC8 was characterized as yeast-like and appeared two celled with average size of 4.30 ± 0.14 µm under scanning electron micrograph. Based on phylogenetic analysis using internal transcribed spacer (ITS) marker, the isolate showed maximum similarity to genus Hortaea. Interestingly, Hortaea sp. strain PMGTC8 (OR527117) exhibited plant-growth-promoting characteristics and caused significantly (p < 0.01) higher germination rate (93.33%), vigor index (964.22), shoot (3.95 cm) and root length (6.38 cm), and fresh weight (0.039 g) of Oryza sativa var. Korgut. This halotolerant black yeast may play a role in nutrition of the plants growing in saline soils. Conclusively, the current findings report for the first time the plant-growth-promoting potential of Hortaea sp. strain PMGTC8 associated with salt crystals of solar saltern, Goa, India for its possible use as bio-fertilizers in saline agriculture soils.

Heterologous Expression and Biochemical Characterization of a New Chloroperoxidase Isolated from the Deep-Sea Hydrothermal Vent Black Yeast Hortaea werneckii UBOCC-A-208029

The initiation of this study relies on a targeted genome-mining approach to highlight the presence of a putative vanadium-dependent haloperoxidase-encoding gene in the deep-sea hydrothermal vent fungus Hortaea werneckii UBOCC-A-208029. To date, only three fungal vanadium-dependent haloperoxidases have been described, one from the terrestrial species Curvularia inaequalis, one from the fungal plant pathogen Botrytis cinerea, and one from a marine derived isolate identified as Alternaria didymospora. In this study, we describe a new vanadium chloroperoxidase from the black yeast H. werneckii, successfully cloned and overexpressed in a bacterial host, which possesses higher affinity for bromide (Km = 26 µM) than chloride (Km = 237 mM). The enzyme was biochemically characterized, and we have evaluated its potential for biocatalysis by determining its stability and tolerance in organic solvents. We also describe its potential three-dimensional structure by building a model using the AlphaFold 2 artificial intelligence tool. This model shows some conservation of the 3D structure of the active site compared to the vanadium chloroperoxidase from C. inaequalis but it also highlights some differences in the active site entrance and the volume of the active site pocket, underlining its originality.

Exploring yeast diversity of dry-salted naturally black olives from Greek retail outlets with culture dependent and independent molecular methods

In the present study, the physicochemical (pH, water activity, moisture content, salt concentration) classical plate counts (total viable counts, yeasts, lactic acid bacteria, Staphylococcus aureus, Pseudomonas spp., Enterobacteriaceae) and amplicon sequencing of naturally black dry-salted olives obtained from different retail outlets of the Greek market were investigated. According to the results, the values of the physicochemical characteristics presented great variability among the samples. Specifically, pH and water activity (a_w) values ranged between 4.0 and 5.0, as well as between 0.58 and 0.91, respectively. Moisture content varied between 17.3 and 56.7 % (g Η₂Ο/100 g of olive pulp), whereas salt concentration ranged from 5.26 to 9.15 % (g NaCl/100 g of olive pulp). No lactic acid bacteria, S. aureus, Pseudomonas spp. and Enterobacteriaceae were detected. The mycobiota consisted of yeasts that were further characterized and identified by culture-dependent (rep-PCR, ITS-PCR, and RFLP) and amplicon target sequencing (ATS). Pichia membranifaciens, Candida sorbosivorans, Citeromyces nyonsensis, Candida etchelsii, Wickerhamomyces subpelliculosus, Candida apicola, Wickerhamomyces anomalus, Torulaspora delbrueckii and Candida versatilis were the dominant species according to ITS sequencing (culture-dependent), while ATS revealed the dominance of C. etchelsii, Pichia triangularis, P. membranifaciens, and C. versatilis among samples. The results of this study demonstrated considerable variability in quality attributes among the different commercial samples of dry-salted olives, reflecting a lack of standardization in the processing of this commercial style. However, the majority of the samples were characterized by satisfactory microbiological and hygienic quality and complied with the requirements of the trade standard for table olives of the International Olive Council (IOC) for this processing style in terms of salt concentration. In addition, the diversity of yeast species was elucidated for the first time in commercially available products, increasing our knowledge on the microbial ecology of this traditional food. Further investigation into the technological and multifunctional traits of the dominant yeast species may result in better control during dry-salting and enhance the quality and shelf-life of the final product.

Hortaea werneckii isolates exhibit different pathogenic potential in the invertebrate infection model Galleria mellonella

Hortaea werneckii is a black yeast with a remarkable tolerance to salt. Most studies have been dedicated to understanding how H. werneckii adapts to hypersaline environments. H. werneckii has an unconventional cell cycle in which it alternates between fission and budding, which is modulated by cell density. Additionally, H. werneckii can cause superficial mycosis of the palm and sole of humans. Here, we determine the impact of salt concentration on the EXF-2000 strain's cell division pattern and morphology by performing timelapse microscopy at different salt concentrations. At low density and no salt, EXF-2000 primarily grows as pseudohyphae dividing mainly by septation. When grown in the presence of salt at a similar concentration to saltwater or hypersaline environments, we observe it grows first by undergoing fission followed by budding at the poles. Then, we examined a collection of 16 isolates in the presence of 0.6M NaCl, including isolates from marine and hypersaline environments and isolates from patients. These isolates exhibit a wide diversity in colony shape and cellular morphology. The isolates grew as yeast, pseudohyphae, and true hyphae, indicating that isolates can exhibit various cell morphologies under similar environmental conditions. We used the insect larvae Galleria mellonella to determine the pathogenic potential of our isolates. We observe that only a subset of isolates can cause death in our model, and there was no correlation between H. werneckii morphology and capacity to cause disease. Taken together, H. werneckii genomic and phenotypic diversity can serve as a model to better understand how phenotypes and pathogenic potential evolve in environmental fungi.

Clonality, inbreeding, and hybridization in two extremotolerant black yeasts

BACKGROUND

The great diversity of lifestyles and survival strategies observed in fungi is reflected in the many ways in which they reproduce and recombine. Although a complete absence of recombination is rare, it has been reported for some species, among them 2 extremotolerant black yeasts from Dothideomycetes: Hortaea werneckii and Aureobasidium melanogenum. Therefore, the presence of diploid strains in these species cannot be explained as the product of conventional sexual reproduction.

RESULTS

Genome sequencing revealed that the ratio of diploid to haploid strains in both H. werneckii and A. melanogenum is about 2:1. Linkage disequilibrium between pairs of polymorphic loci and a high degree of concordance between the phylogenies of different genomic regions confirmed that both species are clonal. Heterozygosity of diploid strains is high, with several hybridizing genome pairs reaching the intergenomic distances typically seen between different fungal species. The origin of diploid strains collected worldwide can be traced to a handful of hybridization events that produced diploids, which were stable over long periods of time and distributed over large geographic areas.

CONCLUSIONS

Our results, based on the genomes of over 100 strains of 2 black yeasts, show that although they are clonal, they occasionally form stable and highly heterozygous diploid intraspecific hybrids. The mechanism of these apparently rare hybridization events, which are not followed by meiosis or haploidization, remains unknown. Both extremotolerant yeasts, H. werneckii and even more so A. melanogenum, a close relative of the intensely recombining and biotechnologically relevant Aureobasidium pullulans, provide an attractive model for studying the role of clonality and ploidy in extremotolerant fungi.

An indigenous inland genotype of the black yeast Hortaea werneckii inhabiting the great pyramid of Giza, Egypt

Within the context of cultural heritage conservation, the biological study of tangible archeological sites is an important task to extend their existence and strengthen the transmission of their cultural value to future generations. In Egypt, a hyper-arid region, a microcolonial fungus with inky black growth was observed on a stone surface in the royal corridor of the Great Pyramid of Giza (King Khufu’s pyramid). The isolate was studied and characterized by microscopic morphometric measurements, evaluation of enzymatic activities, and genotyping techniques. The isolate was identified as Hortaea werneckii, a pleomorphic black yeast that naturally inhabits hypersaline environments and infects human skin. It has been reported from humid temperate, subtropical, and tropical zones, mainly from marine habitats and adjacent areas, and is associated with marine life. Since it was observed in an unusual habitat, it raises the question of its type and origin, whether environmental or clinical. The Egyptian Hortaea werneckii GPS5 isolate was profiled and characterized by adaptive extremophilic tolerance to arid salt stress, low portability to infect human skin, and the capability of solubilizing calcite; besides it was phylogenetically clustered with previous recorded environmental accessions. A profile that matches the biodeterioration fungal agents known as rock-inhabiting fungi, a potential threat to cultural heritage sites that requires attention and prevention plans.

Suggested Sustainable Medical and Environmental Uses of Melanin Pigment From Halotolerant Black Yeast Hortaea werneckii AS1

The marine ecosystem is a complex niche with unique environmental circumstances. Microbial communities from the sea are one of the main origins of compounds with tremendous capabilities. Marine yeasts have the ability to produce secondary metabolites that are architecturally distinct from those found in terrestrial species. Melanin pigment synthesized by marine halotolerant black yeast Hortaea werneckii AS1 isolated from Mediterranean salt lakes in Alexandria, Egypt was found to exert a radical scavenging effect on 2,2-diphenyl-1-picrylhydrazyl (DPPH) with an IC₅₀ of 61.38 μg/ml. Furthermore, it showed no cytotoxicity toward human skin fibroblast cell line (HSF) with an IC₅₀ value above 0.1 mg/ml. The antimicrobial capability of the pigment was revealed against the tested number of bacterial and fungal strains with the highest inhibition zone of 25 mm against Aeromonas sp. and a growth inhibition percentage up to 63.6% against Aspergillus niger. From an environmental impact point of view, the pigment disclosed a heavy metal removal efficiency of 85.7, 84.8, and 81.5% for Pb²⁺, Cd²⁺, and Ni²⁺, respectively, at 100 mg/L metal concentration. The previously mentioned results suggested melanin from H. werneckii AS1 as a promising biocompatible candidate in various medical, cosmetics, pharmaceutical, and environmental applications.

Optimization of melanin pigment production from the halotolerant black yeast Hortaea werneckii AS1 isolated from solar salter in Alexandria

Background

Melanins are one of the magnificent natural pigments synthesized by a wide range of microorganisms including different species of fungi and bacteria. Marine black yeasts appear to be potential prospects for the synthesis of natural melanin pigment. As a result, the goal of this research was to isolate a marine black yeast melanin-producing strain and improve the culturing conditions in order to maximize the yield of such a valuable pigment.

Results

Among five locally isolated black yeast strains, the only one that demonstrated a potent remarkable melanin pigment production was identified using ITS rDNA as Hortaea werneckii AS1. The extracted pigment’s physiochemical characterization and analytical investigation with Ultraviolet-Visible (UV) spectrophotometry, Fourier Transform-Infrared spectroscopy (FTIR), and Scanning Electron Microscope (SEM) confirmed its nature as a melanin pigment. The data obtained from the polynomial model’s maximum point suggested that CaCl₂, 1.125 g/L; trace element, 0.25 ml/L; and a culture volume 225 mL/500 mL at their optimal values were the critical three elements impacting melanin production. In comparison with the baseline settings, the response surface methodology (RSM) optimization approach resulted in a 2.0 - fold improvement in melanin output.

Conclusions

A maximum melanin yield of 0.938 g/L proved the halotolerant H. werneckii AS1 potentiality as a source for natural melanin pigment synthesis ‘when compared to some relevant black yeast strains’ and hence, facilitating its incorporation in a variety of pharmaceutical and environmental applications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02505-1.

Recent developments in the biology and biotechnological applications of halotolerant yeasts

Natural hypersaline environments are inhabited by an abundance of prokaryotic and eukaryotic microorganisms capable of thriving under extreme saline conditions. Yeasts represent a substantial fraction of halotolerant eukaryotic microbiomes and are frequently isolated as food contaminants and from solar salterns. During the last years, a handful of new species has been discovered in moderate saline environments, including estuarine and deep-sea waters. Although Saccharomyces cerevisiae is considered the primary osmoadaptation model system for studies of hyperosmotic stress conditions, our increasing understanding of the physiology and molecular biology of halotolerant yeasts provides new insights into their distinct metabolic traits and provides novel and innovative opportunities for genome mining of biotechnologically relevant genes. Yeast species such as Debaryomyces hansenii, Zygosaccharomyces rouxii, Hortaea werneckii and Wallemia ichthyophaga show unique properties, which make them attractive for biotechnological applications. Select halotolerant yeasts are used in food processing and contribute to aromas and taste, while certain gene clusters are used in second generation biofuel production. Finally, both pharmaceutical and chemical industries benefit from applications of halotolerant yeasts as biocatalysts. This comprehensive review summarizes the most recent findings related to the biology of industrially-important halotolerant yeasts and provides a detailed and up-to-date description of modern halotolerant yeast-based biotechnological applications.

Net Charges of the Ribosomal Proteins of the S10 and spc Clusters of Halophiles Are Inversely Related to the Degree of Halotolerance

Net positive charge(s) on ribosomal proteins (r-proteins) have been reported to influence the assembly and folding of ribosomes. A high percentage of r-proteins from extremely halophilic archaea are known to be acidic or even negatively charged. Those proteins that remain positively charged are typically far less positively charged. Here, the analysis is extended to non-archaeal halophilic bacteria, eukaryotes, and halotolerant archaea. The net charges (pH 7.4) of the r-proteins that comprise the S10-spc operon/cluster from individual microbial and eukaryotic genomes were estimated and intercompared. It was observed that, as a general rule, the net charges of individual proteins remained mostly basic as the salt tolerance of the bacterial strains increased from 5 to 15%. The most striking exceptions were the extremely halophilic bacterial strains, Salinibacter ruber SD01, Acetohalobium arabaticum DSM 5501 and Selenihalanaerobacter shriftii ATCC BAA-73, which are reported to require a minimum of 18% to 21% salt for their growth. All three strains have higher numbers of acidic S10-spc cluster r-proteins than what is seen in the moderate halophiles or the halotolerant strains. Of the individual proteins, only uL2 never became acidic. uS14 and uL16 also seldom became acidic. The net negative charges on several of the S10-spc cluster r-proteins are a feature generally shared by all extremely halophilic archaea and bacteria. The S10-spc cluster r-proteins of halophilic fungi and algae (eukaryotes) were exceptions: these were positively charged despite the halophilicity of the organisms. IMPORTANCE The net charges (at pH 7.4) of the ribosomal proteins (r-proteins) that comprise the S10-spc cluster show an inverse relationship with the halophilicity/halotolerance levels in both bacteria and archaea. In non-halophilic bacteria, the S10-spc cluster r-proteins are generally basic (positively charged), while the rest of the proteomes in these strains are generally acidic. On the other hand, the whole proteomes of the extremely halophilic strains are overall negatively charged, including the S10-spc cluster r-proteins. Given that the distribution of charged residues in the ribosome exit tunnel influences cotranslational folding, the contrasting charges observed in the S10-spc cluster r-proteins have potential implications for the rate of passage of these proteins through the ribosomal exit tunnel. Furthermore, the universal protein uL2, which lies in the oldest part of the ribosome, is always positively charged irrespective of the strain/organism it belongs to. This has implications for its role in the prebiotic context.

Growth and division mode plasticity is dependent on cell density in marine-derived black yeasts

The diversity and ecological contribution of the fungus kingdom in the marine environment remain understudied. A recent survey in the Atlantic (Woods Hole, MA, USA) brought to light the diversity and unique biological features of marine fungi. The study revealed that black yeast species undergo an unconventional cell division cycle, which has not been documented in conventional model yeast species such as Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast). The prevalence of this unusual property is unknown. Here, I collected and identified 65 marine fungi species across 40 genera from the surface ocean water, sediment, and the surface of macroalgae (seaweeds) in the Pacific (Sugashima, Toba, Japan). The Sugashima collection largely did not overlap with the Woods Hole collection and included several unidentifiable species, further illustrating the diversity of marine fungi. Three black yeast species were isolated, two of which were commonly found in Woods Hole (Aureobasidium pullulans and Hortaea werneckii). Surprisingly, their cell division mode was dependent on cell density, and the previously reported unconventional division mode was reproduced only at a certain cell density. For all three black yeast species, cells underwent filamentous growth with septations at low cell density and immediately formed buds at high cell density. At intermediate cell density, two black yeasts (H. werneckii and an unidentifiable species) showed rod cells undergoing septation at the cell equator. In contrast, all eight budding yeast species showed a consistent division pattern regardless of cell density. This study suggests the plastic nature of the growth/division mode of marine-derived black yeast.

Seven Years at High Salinity-Experimental Evolution of the Extremely Halotolerant Black Yeast Hortaea werneckii

The experimental evolution of microorganisms exposed to extreme conditions can provide insight into cellular adaptation to stress. Typically, stress-sensitive species are exposed to stress over many generations and then examined for improvements in their stress tolerance. In contrast, when starting with an already stress-tolerant progenitor there may be less room for further improvement, it may still be able to tweak its cellular machinery to increase extremotolerance, perhaps at the cost of poorer performance under non-extreme conditions. To investigate these possibilities, a strain of extremely halotolerant black yeast Hortaea werneckii was grown for over seven years through at least 800 generations in a medium containing 4.3 M NaCl. Although this salinity is well above the optimum (0.8–1.7 M) for the species, the growth rate of the evolved H. werneckii did not change in the absence of salt or at high concentrations of NaCl, KCl, sorbitol, or glycerol. Other phenotypic traits did change during the course of the experimental evolution, including fewer multicellular chains in the evolved strains, significantly narrower cells, increased resistance to caspofungin, and altered melanisation. Whole-genome sequencing revealed the occurrence of multiple aneuploidies during the experimental evolution of the otherwise diploid H. werneckii. A significant overrepresentation of several gene groups was observed in aneuploid regions. Taken together, these changes suggest that long-term growth at extreme salinity led to alterations in cell wall and morphology, signalling pathways, and the pentose phosphate cycle. Although there is currently limited evidence for the adaptive value of these changes, they offer promising starting points for future studies of fungal halotolerance.

Mycosands: Fungal diversity and abundance in beach sand and recreational waters - Relevance to human health

The goal of most studies published on sand contaminants is to gather and discuss knowledge to avoid faecal contamination of water by run-offs and tide-retractions. Other life forms in the sand, however, are seldom studied but always pointed out as relevant. The Mycosands initiative was created to generate data on fungi in beach sands and waters, of both coastal and freshwater inland bathing sites. A team of medical mycologists and water quality specialists explored the sand culturable mycobiota of 91 bathing sites, and water of 67 of these, spanning from the Atlantic to the Eastern Mediterranean coasts, including the Italian lakes and the Adriatic, Baltic, and Black Seas. Sydney (Australia) was also included in the study. Thirteen countries took part in the initiative. The present study considered several fungal parameters (all fungi, several species of the genus Aspergillus and Candida and the genera themselves, plus other yeasts, allergenic fungi, dematiaceous fungi and dermatophytes). The study considered four variables that the team expected would influence the results of the analytical parameters, such as coast or inland location, urban and non-urban sites, period of the year, geographical proximity and type of sediment. The genera most frequently found were Aspergillus spp., Candida spp., Fusarium spp. and Cryptococcus spp. both in sand and in water. A site-blind median was found to be 89 Colony-Forming Units (CFU) of fungi per gram of sand in coastal and inland freshwaters, with variability between 0 and 6400 CFU/g. For freshwater sites, that number was 201.7 CFU/g (0, 6400 CFU/g (p = 0.01)) and for coastal sites was 76.7 CFU/g (0, 3497.5 CFU/g). For coastal waters and all waters, the median was 0 CFU/ml (0, 1592 CFU/ml) and for freshwaters 6.7 (0, 310.0) CFU/ml (p < 0.001). The results advocate that beaches should be monitored for fungi for safer use and better management.

Evolution and Physiology of Amphibious Yeasts

Since the emergence of the first fungi some 700 million years ago, unicellular yeast-like forms have emerged multiple times in independent lineages via convergent evolution. While tens to hundreds of millions of years separate the independent evolution of these unicellular organisms, they share remarkable phenotypic and metabolic similarities, and all have streamlined genomes. Yeasts occur in every aquatic environment yet examined. Many species are aquatic; perhaps most are amphibious. How these species have evolved to thrive in aquatic habitats is fundamental to understanding functions and evolutionary mechanisms in this unique group of fungi. Here we review the state of knowledge of the physiological and ecological diversity of amphibious yeasts and their key evolutionary adaptations enabling survival in aquatic habitats. We emphasize some genera previously thought to be exclusively terrestrial. Finally, we discuss the ability of many yeasts to survive in extreme habitats and how this might lend insight into ecological plasticity, including amphibious lifestyles. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Salinomyces polonicus: A moderately halophilic kin of the most extremely halotolerant fungus Hortaea werneckii

A clade where the most halotolerant fungus in the world - Hortaea werneckii, belongs (hereafter referred to as Hortaea werneckii lineage) includes five species: Hortaea werneckii, H. thailandica, Stenella araguata, Eupenidiella venezuelensis, and Magnuscella marina, of which the first species attracts increasing attention of mycologists. The species diversity and phylogenetic relationships within this lineage are weakly known. In this study two moderately halophilic black yeast strains were isolated from brine of graduation tower in Poland. Molecular phylogenetic analyses based on the rDNA ITS1-5.8S-ITS2 (=ITS), rDNA 28S D1-D2 (=LSU), and RNA polymerase II (rpb2) sequences showed that the two strains belong to Hortaea werneckii lineage but cannot be assigned to any described taxa. Accordingly, a new genus and species, Salinomyces and Salinomyces polonicus, are described for this fungus. Furthermore, molecular phylogenetic analyses have revealed that Hortaea thailandica is more closely related to S. polonicus than to H. werneckii. A new combination Salinomyces thailandicus is proposed for this fungus.

Towards Genomic Criteria for Delineating Fungal Species

The discussion of fungal species delineation has yet to reach a consensus, despite the advancements in technology, which helped modernise traditional approaches. In particular, the phylogenetic species concept was one of the tools that has been used with considerable success across the fungal kingdom. The fast rise of fungal genomics provides an unprecedented opportunity to expand measuring the relatedness of fungal strains to the level of whole genomes. However, the use of genomic information in taxonomy has only just begun, and few methodological guidelines have been suggested so far. Here, a simple approach of computationally measuring genomic distances and their use as a standard for species delineation is investigated. A fixed threshold genomic distance calculated by the quick and easy-to-use tools Mash and Dashing proved to be an unexpectedly widely applicable and robust criterion for determining whether two genomes belong to the same or to different species. The accuracy of species delineation in an uncurated dataset of GenBank fungal genomes was close to 90%—and exceeded 90% with minimal curation. As expected, the discriminative power of this approach was lower at higher taxonomic ranks, but still significantly larger than zero. Simple instructions for calculation of a genomic distance between two genomes and species similarity thresholds at different k-mer sizes are suggested. The calculation of genomic distance is identified as a powerful approach for delineating fungal species and is proposed—not as the only criterion—but as an additional tool in the versatile toolbox of fungal taxonomy.

Whole Genome Sequencing and Comparative Genome Analysis of the Halotolerant Deep Sea Black Yeast Hortaea werneckii

Hortaea werneckii, an extreme halotolerant black yeast in the order of Capnodiales, was recently isolated from different stations and depths in the Mediterranean Sea, where it was shown to be the dominant fungal species. In order to explore the genome characteristics of these Mediterranean isolates, we carried out a de-novo sequencing of the genome of one strain isolated at a depth of 3400 m (MC873) and a re-sequencing of one strain taken from a depth of 2500 m (MC848), whose genome was previously sequenced but was highly fragmented. A comparative phylogenomic analysis with other published H. werneckii genomes was also carried out to investigate the evolution of the strains from the deep sea in this environment. A high level of genome completeness was obtained for both genomes, for which genome duplication and an extensive level of heterozygosity (~4.6%) were observed, supporting the recent hypothesis that a genome duplication caused by intraspecific hybridization occurred in most H. werneckii strains. Phylogenetic analyses showed environmental and/or geographical specificity, suggesting a possible evolutionary adaptation of marine H. werneckii strains to the deep sea environment. We release high-quality genome assemblies from marine H. werneckii strains, which provides additional data for further genomics analysis, including niche adaptation, fitness and evolution studies.

The beauty and the yeast: can the microalgae Dunaliella form a borderline lichen with Hortaea werneckii?

Lichenized fungi usually develop complex, stratified morphologies through an intricately balanced living together with their algal partners, but several species are known to form only more or less loose associations with algae. These borderline lichens are still little explored although they could inform us about early stages of lichen evolution. We studied the association of the extremely halotolerant fungus Hortaea werneckii with the alga Dunaliella atacamensis, discovered in a cave in the Atacama Desert (Chile), and with D. salina, common inhabitant of saltern brines. D. atacamensis forms small colonies, in which cells of H. werneckii can be frequently observed, while such interaction has not been observed with D. salina. As symbiotic interactions between Dunaliella and Hortaea have not been reported, we performed a series of co-cultivation experiments to inspect whether these species could interact and develop more distinct lichen-like symbiotic structures. We set up co-cultures between axenic strains of Hortaea werneckii (isolated both from Mediterranean salterns and from the Atacama cave) and isolates of D. atacamensis (from the Atacama cave) and D. salina (isolated from Mediterranean salterns). Although we used different growth media and cultivation approaches, bright field and SEM microscopy analyses did not indicate any mutual effects in these experiments. We discuss the implications for fungal algal interactions along the transition from algal exploiters to lichen symbioses.

Unconventional Cell Division Cycles from Marine-Derived Yeasts

Fungi have been found in every marine habitat that has been explored; however, the diversity and functions of fungi in the ocean are poorly understood. In this study, fungi were cultured from the marine environment in the vicinity of Woods Hole, MA, USA, including from plankton, sponge, and coral. Our sampling resulted in 35 unique species across 20 genera. We observed many isolates by time-lapse, differential interference contrast (DIC) microscopy and analyzed modes of growth and division. Several black yeasts displayed highly unconventional cell division cycles compared to those of traditional model yeast systems. Black yeasts have been found in habitats inhospitable to other life and are known for halotolerance, virulence, and stress resistance. We find that this group of yeasts also shows remarkable plasticity in terms of cell size control, modes of cell division, and cell polarity. Unexpected behaviors include division through a combination of fission and budding, production of multiple simultaneous buds, and cell division by sequential orthogonal septations. These marine-derived yeasts reveal alternative mechanisms for cell division cycles that seem likely to expand the repertoire of rules established from classic model system yeasts.